The present invention relates generally to digital radio communications systems, and more specifically to a digital radio receiver for mobile communications systems in which the receiver suffers from multipath fading and interference.
In digital mobile radio, orthogonal modulation techniques are employed for conveying TDM (time division multiplex) signals. To overcome bit errors caused by multipath fading and interference, the maximum likelihood sequence estimation technique is used. A known digital radio receiver includes a linear amplifier for linearly amplifying IF (intermediate frequency) input signal to produce an output X.multidot.cos (2.pi.f.sub.c t .theta.t), where X is the amplitude of the IF input, f.sub.c is the carrier frequency and t is a time variable. An orthogonal detector provides mixing of the amplifier output with a local carrier A.multidot.cos 2.pi.f.sub.c t for detecting the in-phase baseband component (I=aX.multidot.cos .theta.t) and the quadrature-phase baseband component (Q=aX.multidot.sin .theta.t) of the transmitted signal, where A is the amplitude of the local carrier, where a is a constant and the phase component .theta.t contains the information to be conveyed. After conversion to digital form, the I and Q signals are fed into a digital demodulator to recover the transmitted information using the MLSE adaptive equalization technique. In the adaptive equalization process the amplitude component `aX` provides useful information for eliminating bit errors caused by multipath fading and interference. However, with mobile radio the amplitude component `aX` varies significantly with the length of signal propagation path as well as with variations characteristic of the individual path, giving a variation range of 80 dB in a worst condition. Because of the wide dynamic range, the amplitude components `aX` of the baseband signals cannot properly be represented by digital signals of a practical bit length. Although the long-term variations of the i and Q signals may be significantly compressed by the use of an AGC (automatic gain controlled) amplifier instead of the linear IF amplifier, by appropriately proportioning the AGC time constants, the short-term variations are passed through the AGC amplifier and produce quantum noise. Therefore, the prior art demodulator cannot be used for mobile communications system where the distance between source and destination and the signal propagation path are continually varying and signals suffer from multipathfading-related interference.